As is well known, casting is a metal processing method for pouring a metal which has become liquid by being heated at a high temperature (hereinafter, referred to as molten metal) into a die and extracting the cold solidified metal (hereinafter, referred to as work). Casting is classified into sand die casting, metal die casting, and the like according to a die. The metal die casting is further classified into die casting, low pressure casting, gravity casting, and the like according to a magnitude of pressure at which molten metal is injected and an injection speed thereof. Die casting is a metal processing method for injecting molten metal into a metal die formed of a special steel material at a high pressure and a high speed, cooling the molten metal rapidly, and extracting the solidified work.
In die casting, mechanization advances, and productivity is high. Therefore, die casting is widely used in automobile parts, machine parts, and the like.
In a state where there is no release agent, seizure or galling occurs between a work and a metal die. In order to prevent this seizure or the like, prior to injection of molten metal, a release agent or the like is applied to a metal die. A water-soluble release agent having little risk of fire or the like is generally used for this release agent.
In die casting, in order to improve a production efficiency, reduction in cycle time is desired. This reduction in cycle time reduces time for cooling a metal die, and therefore a metal die temperature is rising. Therefore, a release agent having high temperature heat resistance is desired.
In a conventional water-soluble release agent, adhesion of a metal die at a high temperature is not sufficient, and seizure or the like occurs. Therefore, a method for cooling a metal die by applying a large amount of the water-soluble release agent is used. A water-soluble release agent characterized by containing a water-soluble and/or water-dispersible resin having a glass transition point of 30° C. or lower has been proposed (for example, refer to Patent Literature 1).
However, in the water-soluble release agent, in general, the release agent is diluted with water, and a large amount thereof is applied to be used, and therefore an application amount tends to be large. Therefore, application time and drying time are necessary. This causes impediment of reduction in cycle time. Adhesion to a metal die at a high temperature is also insufficient. The water-soluble release agent has large vaporization heat, and cools a metal die rapidly. Therefore, this causes reduction in lifetime of a metal die.
As a conventional oil-based release agent, a die casting oil-based release agent exhibiting excellent releasability even when a metal die temperature is high (for example, refer to Patent Literature 2), a die casting release agent which has improved a working environment due to reduction in generation of oil mist and improved releasability (for example, refer to Patent Literature 3), and an oil-based release agent for die casting, containing a solvent having a flash point of 70 to 170° C. and a silicone oil having a kinematic viscosity of 150 mm2/s or more at 40° C. (for example, refer to Patent Literature 4) have been proposed. However, these oil-based release agents have insufficient lubricity at a high temperature. The die casting oil-based release agent according to Patent Literature 2 is a die casting release agent having a high viscosity, applied to a surface of a metal die by brush application, and therefore has a poor application efficiency, causing impediment of reduction in cycle time.